Dimethyl ether, one of essential basic chemicals in the chemical industry, has high potentials as an aerosol propellant and as a clean fuel. Currently, the potential of dimethyl ether as a clean alternative fuel for internal combustion has been much increased and thus it is in urgent need to develop a more economical process for its manufacture.
There are two general methods of manufacturing dimethyl ether as follows.
The first method relates to a direct synthesis of dimethyl ether from hydrogen and carbon oxides as delineated in the following equation.
      [          Equation      ⁢                          ⁢      1        ]                                                          CO              2                        +                          3              ⁢                              H                2                                              →                                                    CH                3                            ⁢              OH                        +                                          H                2                            ⁢              O                                                            (          1          )                                                          CO            +                          2              ⁢                              H                2                                              →                                    CH              3                        ⁢            OH                                                (          2          )                                                          2            ⁢                          CH              3                        ⁢            OH                    →                                                    CH                3                            ⁢                              OCH                3                                      +                                          H                2                            ⁢              O                                                            (          3          )                                                          CO            +                                          H                2                            ⁢              O                                →                                    CO              2                        +                          H              2                                                            (          4          )                    
The manufacturing method of dimethyl ether directly from a gaseous mixture of H2/CO/CO2 using methanol catalysts and dehydration catalysts in fixed bed reactors are described in East German Pat. No. 291,937, U.S. Pat. No. 5,254,596, etc. Catalysts for methanol synthesis that can be used in the above Equation 1 include ZnO/Al2O3 and CuO/ZnO/Al2O3, and zeolites can be used as a dehydrating catalyst (U.S. Pat. No. 4,536,485, Korean Pat. No. 228,748).
The second method involves dehydration of methanol as delineated in the Equation 2.
      [          Equation      ⁢                          ⁢      2        ]              2      ⁢              CH        3            ⁢      OH        →                            CH          3                ⁢                  OCH          3                    +                        H          2                ⁢        O            
The synthesis of dimethyl ether via dehydration of methanol as described in the Equation 2 is achieved at 250-450° C. using a solid catalyst. Gamma-alumina (Japanese Laid-Open Pat. Appl. No. 1984-16845) and silica-alumina (Japanese Laid-Open Pat. Appl. No. 1984-42333) are ordinarily used as solid catalysts in a synthesis of dimethyl ether.
The conversion of methanol into dimethyl ether is carried out using an acid catalyst. The activity and selectivity of a catalyst vary depending on the acidity of an acid catalyst because the conversion of methanol into dimethyl ether is an intermediate step in hydrocarbon synthesis.
For instance, in the presence of a catalyst bearing mainly strong acid sites, methanol, after it is converted into dimethyl ether, is proceeded further to generate hydrocarbons as side products. On the other hand, in the presence of a catalyst bearing mainly weak acid sites, the activity of the catalyst becomes low and thus results in insufficient conversion of methanol into dimethyl ether.
The use of a hydrophobic zeolite such as USY, Mordenite, ZSM family, Beta, and others in dehydration reaction of methanol shows a stronger catalytic activity than the use of gamma-alumina at low temperature. However, the strong acidity of such zeolites produce hydrocarbons and cokes as side products during the conversion of methanol into dimethyl ether thereby reducing selectivity. According to the results of the present inventors, conventional H-USY, H-ZSM-5, and H-Beta zeolites are too acidic they produce hydrocarbons as by-products such as methane, ethane, and propane. Further, the hydrocarbon by-products are low molecular weight alkanes with little value and also lead to the deactivation of catalysts via coking.
A method for synthesizing dimethyl ether using crude methanol containing 5-50 mole % water in order to inhibit hydrocarbon production by hydrophobic zeolites has been disclosed by the present inventors (Korean Pat. Appl. No. 2004-51032; U.S. Pat. No. 6,740,783).